Single P-N Junction Tandem Photovoltaic Device

ABSTRACT

A single P-N junction solar cell is provided having two depletion regions for charge separation while allowing the electrons and holes to recombine such that the voltages associated with both depletion regions of the solar cell will add together. The single p-n junction solar cell includes an alloy of either InGaN or InAlN formed on one side of the P-N junction with Si formed on the other side in order to produce characteristics of a two junction (2J) tandem solar cell through only a single P-N junction. A single P-N junction solar cell having tandem solar cell characteristics will achieve power conversion efficiencies exceeding 30%.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of and claims priority to U.S. Utilitypatent application Ser. No. 11/777,963, entitled “Single P-N JunctionTandem Photovoltaics,” filed on Jul. 13, 2007, which claims priority toU.S. Provisional Patent Application Ser. No. 60/945,281, entitled“Single P-N Junction Tandem Photovoltaics,” filed on Jun. 20, 2007, thecontents of both of which are incorporated herein by reference in theirentireties.

STATEMENT OF GOVERNMENTAL INTEREST

The invention described and claimed herein was made in part utilizingfunds supplied by the U.S. Department of Energy under Contract No.DE-AC02-05CH11231. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosure relates to solar cells are more particularly to a singlejunction tandem solar cell.

2. Background Discussion

Solar or photovoltaic cells are semiconductor devices having P-Njunctions which directly convert radiant energy of sunlight intoelectrical energy. Conversion of sunlight into electrical energyinvolves three major processes: absorption of sunlight into thesemiconductor material; generation and separation of positive andnegative charges creating a voltage in the solar cell; and collectionand transfer of the electrical charges through terminals connected tothe semiconductor material. A single depletion region for chargeseparation typically exists in the P-N junction of each solar cell.

Current traditional solar cells based on single semiconductor materialhave an intrinsic efficiency limit of approximately 31%. A primaryreason for this limit is that no one material has been found that canperfectly match the broad ranges of solar radiation, which has a usableenergy in the photon range of approximately 0.4 to 4 eV. Light withenergy below the bandgap of the semiconductor will not be absorbed andconverted to electrical power. Light with energy above the bandgap willbe absorbed, but electron-hole pairs that are created quickly lose theirexcess energy above the bandgap in the form of heat. Thus, this energyis not available for conversion to electrical power.

Higher efficiencies have been attempted to be achieved by using stacksof solar cells with different band gaps, thereby forming a series ofsolar cells, referred to as “multijunction,” “cascade,” or “tandem”solar cells. Tandem solar cells are the most efficient solar cellscurrently available. Tandem cells are made by connecting a plurality(e.g., two, three, four, etc.) P-N junction solar cells in series.Tandem cells are typically formed using higher gap materials in the topcell to convert higher energy photons, while allowing lower energyphotons to pass down to lower gap materials in the stack of solar cells.The bandgaps of the solar cells in the stack are chosen to maximize theefficiency of solar energy conversion, where tunnel junctions are usedto series-connect the cells such that the voltages of the cells sumtogether. Such multijunction solar cells require numerous layers ofmaterials to be formed in a complex, stacked arrangement.

SUMMARY

In accordance with one or more embodiments, a single P-N junction solarcell is provided having multiple regions for charge separation whileallowing the electrons and holes to recombine such that the voltagesassociated with both depletion regions of the solar cell will addtogether. In one or more embodiments, the conduction band edge (CBE) ofa top layer in the solar cell is formed to line up with the valence bandedge (VBE) of a lower layer in the solar cell.

In accordance with one or more embodiments, a solar cell is providedhaving an alloy of either InGaN or InAlN formed on one side of the P-Njunction with Si formed on the other side in order to producecharacteristics of a two junction (2J) tandem solar cell through asingle P-N junction. In one embodiment, an In_(1-x)Ga_(x)N alloy isutilized, while in another embodiment In_(1-x)Al_(x)N is utilized. Asingle P-N junction solar cell formed in accordance with one or moreembodiments will achieve power conversion efficiencies exceeding 30%.

DRAWINGS

The above-mentioned features and objects of the present disclosure willbecome more apparent with reference to the following description takenin conjunction with the accompanying drawings wherein like referencenumerals denote like elements and in which:

FIG. 1 is a block diagram representation of a single P-N junction tandemsolar cell in accordance with one or more embodiments of the presentdisclosure.

FIG. 2 is a more detailed perspective view of FIG. 1 showing the variousregions in a single P-N junction tandem solar cell in accordance withone or more embodiments of the present disclosure.

FIG. 3 is a graphical illustration of a band diagram for theheterojunction of a single P-N junction tandem solar cell in accordancewith one or more embodiments of the present disclosure.

FIGS. 4A and 4B are graphical illustrations of the calculated (a) banddiagram and (b) electron and hole concentrations for the heterojunctionof a single P-N junction tandem solar cell in accordance with one ormore embodiments of the present disclosure.

FIGS. 5A and 5B are graphical illustrations of the calculated banddiagram for the heterojunction of a single P-N junction tandem solarcell with (a) counterdoping and (b) an insulating interlayer at theinterface in accordance with one or more embodiments of the presentdisclosure.

FIGS. 6A and 6B are block diagram representations of a single P-Njunction tandem solar cell having heavily counter-doped regions inaccordance with one or more embodiments of the present disclosure. FIG.6C is a block diagram representation of a single P-N junction tandemsolar cell having an insulating interlayer in accordance with one ormore embodiments of the present disclosure.

DETAILED DESCRIPTION

In general, the present disclosure includes a single P-N junction tandemphotovoltaic device. Certain embodiments of the present disclosure willnow be discussed with reference to the aforementioned figures, whereinlike reference numerals refer to like components.

Referring now to FIG. 1, a block diagram illustration of a single P-Njunction tandem solar cell 100 is shown generally in accordance with oneor more embodiments. One of the layers 102 and 104 is formed as a p-typematerial while the other of the layers 102 and 104 is formed as ann-type material, such that a single P-N junction 105 exists between thelayers 102 and 104. Each of the layers 102 and 104 can also be describedand/or formed as its own subcell within the solar cell 100. In one ormore embodiments, the conduction band edge (CBE) of the top layer 102 inthe solar cell is formed to line up with the valence band edge (VBE) ofthe lower layer 104 in the solar cell 100. In one embodiment, the solarcell 100 includes a layer 102 of a Group III-nitride alloy and a Silayer 104. Electrical contacts 106 and 108 are formed, respectively, onthe top of or otherwise coupled to the Group III-nitride alloy layer 102and on the bottom of or otherwise coupled to the Si layer 104. In one ormore embodiments, the top electrical contact 106 should be formed from asubstantially transparent conductive material so as to allow solarradiation to travel past the electrical contact 106 to enter into thesolar cell 100, such as by forming the contact 106 as Indium-Tin-Oxideor other suitable substantially transparent conductive material or agrid of other metal layers. The electrical contacts 106 and 108 areformed in accordance with methods known to those skilled in the art ofmanufacturing solar cells.

In one embodiment, the layer 102 is an alloy of In_(1-x)Ga_(x)N, where0≦x≦1, having an energy bandgap range of approximately 0.7 eV to 3.4 eV,providing a good match to the solar energy spectrum. In anotherembodiment, the layer 102 is an alloy of In_(1-x)Al_(x)N, where 0≦x≦1,having an energy bandgap range of approximately 0.7 eV to 6.0 eV, alsoproviding a good match to the solar energy spectrum. In one or moreembodiments, the layer 102 is grown by molecular beam epitaxy creatingcrystals with low electron concentrations and high electron mobilities,while it is understood that other formation methods can further beutilized. For ease of description in the various embodiments describedherein, the layer 102 will be referred to as InGaN layer 102, while itis understood that InAlN can interchangeably be substituted in place ofInGaN in the various embodiments described herein.

In one or more embodiments, the InGaN layer 102 is formed as a p-typelayer by doping the InGaN layer 102 with a p-type dopant, such asmagnesium (Mg), while a thin Si interface layer is counter-doped with ap-type dopant such as Boron (B), Aluminum (Al), Gallium (Ga) or Indium(In). The rest of the Si layer 104 is formed as an n-type layer bydoping the Si layer 104 with an n-type dopant, such as phosphorous (P),arsenic (As) or antimony (Sb). Typical doping levels for n-type andp-type layers range from 10¹⁵ cm⁻³ to 10¹⁹ cm⁻³. The actual dopinglevels depend on other characteristics of the layers 102 and 104 of thesolar cell 100 and can be adjusted within and outside of this range tomaximize the efficiency. It is understood that the P-N junction 105 canalso be formed by doping the InGaN layer 102 with an n-type dopant anddoping the Si layer 104 with a p-type dopant. Silicon is commonly usedas an n-type dopant and magnesium as a p-type dopant in InGaN.

As grown, undoped InGaN films are generally n-type, where in oneembodiment the InGaN layer 102 can be doped with Mg acceptors so thatthe InGaN layer 102 behaves as a p-type. In one specific embodiment, aMg p-type dopant is used in alloy of In_(y)Ga_(1-y)N where 0.67≦y≦0.95.

While the P-N junction 105 can be simply formed as represented in FIG. 1with an InGaN layer 102 positioned against a Si layer 104. In actuality,a plurality of depletion regions will be formed across the P-N junction105 when the junction 105 is in thermal equilibrium and in a steadystate. Electrons and holes will diffuse into regions with lowerconcentrations of electrons and holes, respectively. Thus, the excesselectrons in the n-type Si layer 104 will diffuse into the P-side of theP-N junction 105 while the excess holes in the p-type InGaN layer 102will diffuse into the N-side of the P-N junction 105. As illustrated inFIG. 2, this will create an InGaN depletion region 110 in the InGaNlayer 102 adjacent to the P-N junction 105 and a Si depletion region 112in the Si layer 104 adjacent to the P-N junction 105.

When the solar cell 100 is exposed to solar energy, energy transfersfrom photons in the solar energy to the solar cell 100 when the layers102 and 104 absorb lightwaves that contain the same amount of energy astheir bandgap. A bandgap is the energy required to push an electron froma material's valence band to its conduction band. Based upon anexperimental measurement of a 1.05±0.25 eV valence band offset betweenInN and GaN and the known electron affinity of GaN, InN is predicted tohave an electron affinity of 5.8 eV, the largest of any knownsemiconductor. Forming the layer 102 as an alloy of InGaN or InAlNallows a wide bandgap tuning range, 0.7 to 3.4 eV for InGaN and 0.7 to6.0 eV for InAlN.

By aligning the conduction band of one of the layers 102 or 104 with thevalence band of the other one of the layers 102 or 104, a low resistancetunnel junction is produced between the layers 102 and 104. Theconduction band edge 120 and valence band edge 122 positions of InGaNand InAlN are illustrated in FIG. 3, where the dotted lines 114 and 116indicate the compositions (e.g., approximately In_(0.7)Al_(0.3)N orIn_(0.5)Ga_(0.5)N) that align the conduction band of InGaN and InAlN,respectively, with the valence band of Si. A composition with slightlymore Ga or Al will align the conduction band of InGaN/InAlN with thevalence band of Ge. As shown in FIG. 3, the electron affinity (energyposition of the conduction band minimum (CBM) with respect to the vacuumlevel) can also be tuned over a wide range, 5.8 eV to 2.1 eV in InAlNand 5.8 eV to 4.2 eV in InGaN. In one embodiment, for the composition ofapproximately Al_(0.3)In_(0.7)N or In_(0.45)Ga_(0.55)N, the conductionband of AlInN/InGaN can be made to align with the valence band of Si,creating the conditions for a very low resistance tunnel between thelayers 102 and 104 without the requirement of additional heavily dopedlayers as typically required in previous multijunction solar cells,which greatly simplifies the design of the present single junctiontandem solar cell 100 over previous multi-junction solar cells.

The solar cell 100 having a single P-N junction 105 between the p-typelayer 102 (InGaN or InAlN) and the n-type Si layer 104 provides: (1) twodepletion regions for charge separation and (2) a junction 105 thatallows electrons and holes to recombine such that the voltages generatedfrom the solar energy in both of the layers 102 and 104 will addtogether. These types of observations have only previously beenattainable in multijunction tandem solar cells with tunnel junctionlayers and never previously attainable using only a single P-N junction.

The single p-InGaN/n-Si heterojunction of the solar cell 100 behaves ina fundamentally different manner than a usual P-N semiconductorheterojunction. In a normal P-N junction, holes are depleted on thep-type side and electrons are depleted on the n-type side, creating asingle depletion region. However, the present p-InGaN/n-Siheterojunction (or p-InAlN/n-Si heterojunction) formed in accordancewith one or more embodiments produces two depletion regions. Underillumination, both of these depletion regions can separate charge, suchthat a single p-InGaN/n-Si or p-InAlN/n-Si heterojunction functions as atwo-junction tandem solar cell. Further, at the junction 105 between thelayers 102 and 104, there is type inversion (excess electrons on theInGaN side of the junction 105 and excess holes on the Si side of thejunction 105), thereby creating the InGaN depletion region 110 and theSi depletion region 112. This type inversion provides a more efficientelectron-hole annihilation and series connection of the layers 102 and104.

For one embodiment having a p-type In_(0.45)Ga_(0.55)N layer 102 and ann-type Si layer 104, the calculated band diagram and electron and holeconcentrations for such a p-InGaN/n-Si heterojunction tandem solar cellare respectively illustrated in FIGS. 4A and 4B. The two depletionregions 130 and 132 can be seen in FIG. 4A that correspond to depletionregions 110 and 112 shown in FIG. 2. A InGaN bandgap of 1.8 eV wasobtained by specifying the composition, which is close to the ideal fora top layer 102 matched to a bottom Si layer 104 having a band gap=1.1eV in terms of maximum power conversion efficiency.

Under illumination, photons with energies greater than the 1.8 eV bandgap of the In_(0.45)Ga_(0.55)N layer 102 create electron-hole pairs inthe InGaN layer 102. The Si layer 104 absorbs light with energiesbetween 1.1 and 1.8 eV and light with energy>1.8 eV that is not absorbedby the top InGaN layer 102. Doping in both of the layers 102 and 104 canbe adjusted to change the size of the depletion regions 130 and 132.Efficient electron and hole recombination occurs at the InGaN/Sijunction 105 such that under illumination, holes will go to the surfaceof the InGaN layer 102 and electrons will go into the Si layer 104. Athin (˜25 nm) heavily doped p++ layer can be used to provide an ohmiccontact to the InGaN surface.

The depletion regions 130 and 132 are similar to “Schottky-like”depletion regions found in semiconductor materials, such that these twodepletion regions 130 and 132 should achieve efficiency of in the solarcell 100 of approximately 42% for unconcentrated sunlight, similar tothe efficiency achieved by 2J tandem cells.

In one or more embodiments, the dark current (i.e., the output currentof the solar cell 100 when no light is acting as an input) can bereduced by heavy counter-doping (i.e., p⁺⁺ counter-doping 140 in then-type layer 104 as illustrated in FIG. 6A or n⁺⁺ counter-doping 142 inthe p-type layer 102 as illustrated in FIG. 6B) near the interfacebetween at least one of the layers 102, 104 and the respective one ofthe electrical contacts 106, 108. This will also increase the opencircuit voltage and efficiency of the solar cell 100. Referring to FIG.5A, a band diagram for a p-InGaN/n-Si heterojunction tandem solar cellis illustrated in which n⁺⁺ counter-doping (e.g., a 10 nm layer ofn⁺⁺9×10¹⁷) has been utilized in the p-type layer 102 adjacent toelectrical contact 106, where line 140 represents CBE (eV), line 142represents VBE (eV) and line 144 represents E_(F).

In one or more embodiments, the dark current can be reduced and the opencircuit voltage increased through the use of a thin insulatinginterlayer 144 (e.g., a thin layer of GaN) formed between the layers 102and 104, as illustrated in FIG. 6C. The interlayer 144 will serve toincrease the barrier for hole leakage from the p-InGaN layer 102 intothe n-Si layer 104 while preventing electron leakage from the n-Si layer104 into the p-InGaN layer 102. Referring to FIG. 5B, a band diagram fora p-InGaN/n-Si heterojunction tandem solar cell is illustrated in whicha thin 5 nm GaN interlayer 144 has been utilized between the p-InGaNlayer 102 and the n-Si layer 104.

Both of the approaches associated with reducing dark current using heavycounter-doping or a thin insulating layer illustrated in FIGS. 5A and 5Bwill increase the barrier against electron and hole leakage by about 0.1to 0.2 eV compared designs without such features.

In order to form a tandem photovoltaic device using a single P-Njunction, the conduction band minimum (CBM) in the upper layer 102 ofthe solar cell 100 is formed to be substantially aligned with or lowerin energy with respect to the vacuum level than the valence band maximum(VBM) of the lower layer 104 of the solar cell 100. The presentdisclosure allows a solar cell having the efficiency characteristics ofa two-junction tandem solar cell to be made with a very simple singleP-N junction design. By simply forming a p-InGaN layer 102, which can bethin (<0.5 μm), over a bottom n-Si layer 104, a tandem solar cell 100can be produced with an efficiency above that of the best currentlyproduced single junction Si solar cells. In one or more embodiments, theSi layer 104 can be formed using polycrystalline, multicrystalline oreven amorphous Si. Such a tandem solar cell 100 can be produced withincreased efficiency and lower costs compared to previously-known Sitechnology, which could revolutionize photovoltaics manufacturing.

1. A solar cell, comprising: a p-type layer comprising InAlN; an n-type layer; a single p-n junction between the p-type layer and the n-type layer; a charge separation region for charge separation under illumination formed in the p-type layer having a first band gap; and a charge separation region for charge separation under illumination formed in the n-type layer having a second band gap, wherein the second band gap is different from the first band gap, wherein each of the charge separation regions in the p-type and n-type layers are formed to separate charge under illumination separately from one another, wherein the solar cell includes a plurality of charge separation regions for charge separation associated with the single p-n junction.
 2. The solar cell of claim 1, wherein the solar cell includes two depletion regions.
 3. The solar cell of claim 1, wherein the n-type layer comprises Si.
 4. The solar cell of claim 1, wherein a conduction band of one of the p-type and n-type layers substantially aligns with a valence band in the other of the p-type and n-type layers to form the p-n junction as a low resistance tunnel junction between the p-type layer and the n-type layer.
 5. The solar cell of claim 1, wherein voltages produced in each of the charge separation regions will add together to produce a combined output voltage for the solar cell.
 6. The solar cell of claim 1, wherein the InAlN layer is an alloy comprising In1xAlxN, where 0≦x≦1.
 7. The solar cell of claim 6, wherein x is approximately 0.3.
 8. The solar cell of claim 1, further comprising: a first electrical contact coupled to the p-type layer; and a second electrical contact coupled to the n-type layer.
 9. The solar cell of claim 8, further comprising a counter-doped region in at least one of the p-type layer and the n-type layer respectively adjacent to at least one of the first and second electrical contacts.
 10. The solar cell of claim 1, further comprising an insulating interlayer between the p-type layer and the n-type layer.
 11. The solar cell of claim 1, wherein one of the p-type and n-type layers possesses a band gap that is larger than a band gap of the other of the p-type and n-type layers such that each of the charge separation regions absorb different respective portions of the solar spectrum, wherein a conduction band edge of one of the p-type and n-type layers including the charge separation region with the larger band gap is substantially aligned with a valence band edge of the other of the p-type and n-type layers including the charge separation region with the smaller band gap.
 12. A single junction solar cell, comprising: a single p-n junction between a p-type subcell comprising InAlN and an n-type subcell having a plurality of independent charge separation regions for charge separation, each of the charge separation regions are located in a material possessing a different respective band gap such that each charge separation region will absorb different respective portions of the solar spectrum under illumination, wherein a conduction band of one of the p-type and n-type subcells substantially aligns with a valence band in the other of the p-type and n-type subcells to form the p-n junction as a low resistance tunnel junction between the p-type and the n-type subcells.
 13. The solar cell of claim 11, wherein the p-type subcell is a layer of p-type material.
 14. The solar cell of claim 11, wherein the n-type subcell is a layer of n-type material.
 15. The solar cell of claim 11, wherein one of the p-type and n-type layers possesses a band gap that is larger than a band gap of the other of the p-type and n-type layers such that each of the charge separation regions absorb different respective portions of the solar spectrum, wherein a conduction band edge of one of the p-type and n-type subcells including the first charge separation region is substantially aligned with a valence band edge of the other of the p-type and n-type subcells including the second charge separation region.
 16. A method of forming a single junction solar cell, comprising: arranging a p-type layer comprising InAlN adjacent to an n-type layer to form a single p-n junction between the p-type layer and the n-type layer; and forming separate charge separation regions for charge separation on both sides of the single p-n junction for independently performing charge separation under illumination, wherein a conduction band of one of the p-type and n-type layers substantially aligns with a valence band in the other of the p-type and n-type layers to form the p-n junction as a low resistance tunnel junction between the p-type layer and the n-type layer.
 17. The method of claim 16, further comprising coupling a first electrical contact coupled to the p-type layer and a second electrical contact coupled to the n-type layer.
 18. The method of claim 16, further comprising forming the n-type layer from a material comprising Si.
 19. The method of claim 17, further comprising forming a counter-doped region in at least one of the p-type layer and the n-type layer respectively adjacent to at least one of the first and second electrical contacts.
 20. The method of claim 16, further comprising forming each of the p-type layer and the n-type layer to possess a different band gap such that each of the charge separation regions absorb different respective portions of the solar spectrum. 